Microfibrillated plant fibers are widely known as reinforcing materials for resins due to their excellent properties, such as light weight, high strength, high modulus, and low linear thermal expansion. However, microfibrillated plant fibers, which are very highly hydrophilic, have poor compatibility with highly hydrophobic resins, such as polypropylene and polyethylene, and cannot be uniformly mixed by mere mechanical kneading using a twin-screw extruder or the like, and the resulting composite material does not have good mechanical properties.
Therefore, many attempts have been made to improve dispersibility of microfibrillated plant fibers in resin by hydrophobically modifying the microfibrillated plant fibers, as disclosed in Patent Literature (PTL) 1 to 3.
Patent Literature 1 discloses a hydrophobically modified microfibrillated plant fiber obtained by esterifying pulp with acid halide and subjecting the resulting hydrophobically modified pulp to bead milling in an aqueous medium. Further, the obtained hydrous hydrophobically modified microfibrillated plant fiber is kneaded with a resin to produce a composite material composed of the hydrophobically modified microfibrillated plant fiber and resin.
When such a hydrous hydrophobically modified microfibrillated plant fiber is mixed with a resin, the hydrophobically modified microfibrillated plant fiber is not fully dispersed in the resin due to the water it contains, and the resulting composite material does not have sufficient mechanical properties, such as tensile strength and modulus.
Patent Literature 2 discloses modifying cellulosic fibers in an aqueous system. Because microfibrillated plant fibers are very highly cohesive, it is difficult to completely uniformly disperse the microfibrillated plant fibers even in water that has relatively high compatibility. Therefore, even when either cellulose fibers or microfibrillated plant fibers are hydrophobized in an aqueous system, only a slight surface of the fibers can be hydrophobically modified. Accordingly, when shearing force is applied during kneading, hydrophobically unmodified cellulose surfaces are formed. This makes it difficult to fully disperse the cellulose fibers in resin. Further, because the hydrophobically unmodified cellulose surfaces become locations of connection failure and are susceptible to destruction, a composite material with excellent mechanical strength cannot be obtained.
In Patent Literature 3, microfibrillated plant fibers are modified in toluene, which is poor in terms of cellulose swellability, and the modification is not performed in a cellulose swelling solvent as used in the present invention. Therefore, uniform modification of the microfibrillated plant fiber surface is difficult. Accordingly, as in PTL 2, the microfibrillated plant fibers are not fully dispersed in a resin, thus failing to provide a composite material with excellent mechanical properties.
Therefore, when a highly hydrophobic thermoplastic resin, such as polyethylene or polypropylene, is used to produce a molding material comprising microfibrillated plant fibers, the microfibrillated plant fibers are poorly dispersed in the resin, and enhanced mechanical strength is very difficult to achieve.